1. FIELD OF THE INVENTION
The present invention generally relates to an internal combustion engine in which the cylinder or cylinders is disposed substantially horizontally, the cylinder having intake and exhaust valves at its head. More specifically, this invention pertains to a lubricating system incorporated in the foregoing internal combustion engine which is equipped with a breather passage and an oil-returning passage providing communication between a crank chamber for storing a lubricating oil and an air-oil separating chamber located on the side of the cylinder head, without any special lubricating pump being included for lubricating a valve drive mechanism.
2. DESCRIPTION OF THE PRIOR ART
The above-mentioned lubricating system is arranged in the following manner. The oil mist produced within the crank chamber is subjected to the positive pressure which is generated by the reciprocal motion of the piston within the crank chamber, and the oil mist is fed to the air-oil separating chamber through the breather passage. After the mist has lubricated a valve operating mechanism within the air-oil separating chamber, the oil component is accumulated therein. The thus-accumulated oil is subjected to the force of gravity and the negative pressure produced within the crank chamber and is caused to flow into the crank chamber through the oil-returning passage. Since such a system has a very simple structure, it has been used in various small-sized overhead valve engines, and Japanese Utility Model Examined Publication No. 6585/1985 discloses such a lubricating system.
The above-described prior-art system can be adapted to a vertical cylinder engine without difficulty. However, if the prior art is to be applied to an engine having a substantially horizontal cylinder, when the pressure within the crank chamber changes from negative to positive, the positive pressure acts not only on the breather passage but also the oil-returning passage. As a result, there is a risk of the oil being forced back to the air-oil separating chamber while flowing in the oil-returning passage.
In addition, the prior-art system involves difficulties in that a large quantity of lubricating oil is prone to flow back from the crank chamber to the air-oil separating chamber through the oil-returning passage when an operator inclines the cylinder head downwardly when wishing to inspect the engine or other working parts coupled thereto when the engine is inoperative. After this large quantity of oil has entered the air-oil separating chamber, it in turn flows through a breather port into an air cleaner, a carburetor, a combustion chamber and a muffler, thereby raising such problems as difficulty in starting, damage to or contamination of air cleaner elements, and the generation of white smoke.
One type of internal combustion engine is known in which a breather is extended directly out of the engine through its breather port. Should the cylinder head of the engine be inclined downwardly, a large quantity of lubricating oil within the air-oil separating chamber is discharged out of the engine, thereby causing such problem as damage to or contamination of the engine.